The ability to reproduce very small dimensions is important in fabricating large scale integrated circuits (“ICs”) on, for example, silicon wafers or substrates. One way to increase circuit density on a wafer or substrate is to increase the resolution capabilities of photoresist used to form photoresist film patterns. Thus, as the integration degree of ICs becomes higher, finer photoresist film patterns are required.
Photoresists are either positive working or negative working. In using a negative working photoresist (“negative photoresist”), radiation-exposed areas harden, and form image areas of the photoresist after unexposed areas are removed using a developer. In using a positive working photoresist (“positive photoresist”), however, radiation-exposed areas are non-image areas, and are rendered soluble in aqueous alkali developers. Positive photoresists have been found to be capable of higher resolution than negative photoresists, and as a result, positive photoresists have mostly replaced negative photoresists in fabricating ICs.
The production of positive photoresists is well known in the art, as disclosed, for example, in U.S. Pat. Nos. 3,666,473; 4,115,128; and 4,173,470. As disclosed therein, a typical positive photoresist formulation comprises aqueous alkali soluble polyvinyl phenol or phenol formaldehyde novolak resins, and light sensitive materials (“photosensitizers”)—usually a substituted naphthoquinone diazide compound. In use, the resins and photosensitizers are dissolved in an organic solvent, and are applied as a thin film coating to a substrate suitable for a particular application.
As is known, the resin component of such a positive photoresist formulation is soluble in an aqueous alkaline solution, but the photosensitizer is not. Upon image-wise exposure of the coated substrate to actinic radiation, exposed areas of the coating are rendered more soluble than unexposed areas. This difference in solubility causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution (while the unexposed areas are substantially unaffected), thus producing a positive image on the substrate. The uncovered substrate is thereafter subjected to an etching process. In use, the photoresist coating protects covered areas of the substrate from the etchant. Thus, a pattern can be created on the substrate that corresponds to the pattern of the mask or template used to create selective exposure patterns on the coated substrate prior to development.
An optimally obtainable microlithographic resolution for an exposure pattern is determined by the wavelength of the radiation used for selective irradiation. As a result, the resolution capacity that can be obtained with conventional UV microlithography has its limits, and in order to sufficiently resolve optically small structural elements, wavelengths shorter than UV radiation must be utilized. However, UV photoresist materials used today are not suitable for use with radiation at wavelengths of 157 nm, 193 nm or 248 nm. This is because materials based on phenolic resins as a binding agent (particularly novolak resins or polyhydroxystyrene derivatives) have too high an absorption at such wavelengths, and cannot be imaged through films of the thicknesses needed to fabricate ICs (for example, polyhydroxystyrene based photoresists can be used in top surface imaging applications in which only a very thin (˜500 Å) layer of photoresist is required to be transparent at the wavelength of an ArF laser, i.e., λ˜193 nm). Such high absorption results in side walls of developed photoresist structures that do not form vertical profiles. Rather the side walls form an oblique angle with respect to the substrate, and this causes poor optical resolution characteristics at these short wavelengths.
To solve the above-described problem, chemical amplification photoresists have been developed that have been found to provide superior resolution. For 157 nm, 193 nm, and 248 nm photoresist technologies, chemical amplification is used to achieve high “photo” speed at the low exposure energies provided by the selected wavelength sources. The basic photoresist design concept for such photoresists is to start with a resin that has good transparency at the selected wavelength—the resin must also be highly insoluble in an aqueous base developer chemistry. To make the resin insoluble in such a developer, dissolution inhibitors (sometimes called blocking or protecting groups) are attached to the resin. These protecting groups are usually very large, or bulky, molecules that are attached to the resin via bonds that can be easily cleaved. In most advanced photoresist systems, there are usually several types of molecules attached to the resin in addition to the dissolution inhibitors. These include molecules that are intended to enhance the etch resistance of the material, as well as molecules that are intended to help with lithographic performance. All of these molecules are attached to the resin via a link that is easily cleaved.
Chemical amplification is achieved by adding a small amount of a photo-acid generator (PAG). This is a compound that generates a proton (H+) whenever it is exposed at the appropriate wavelength. These compounds are usually onium salts, such as sulfonium salts, but they can be any one of a number of suitable compounds. Whenever the PAG is exposed, and the proton is generated, the proton cleaves the nearest available bond between the resin and a dissolution inhibitor. This cleaving reaction generates another proton, which proton cleaves the next nearest bond, and so on. This is sometimes referred to as a de-protection reaction.
This de-protection reaction can occur during an optical exposure, for low activation energy photoresists, or during a subsequent post-exposure bake (PEB), for high activation energy photoresists. The result of the de-protection reaction is the formation of an acid, which acid is soluble in an aqueous base developer. In addition, as a result of cleaving the link between the resin and the blocking group, the blocking molecule usually leaves the photoresist as a volatile. In accordance with the above-described process, the chemically amplified photoresist can be fully exposed even though the incident optical exposure dose is very low, for example, on the order of about 10 to about 20 mJ/cm2. Typically, the chemistry of such chemically amplified 157 nm, 193 nm or 248 nm photoresists is based on polymers such as, but not limited to, acrylates, cyclic olefins with alicyclic groups, and hybrids of the aforementioned polymers which lack aromatic rings. FIG. 1 shows an example of a methacrylate type and a hybrid type chemically amplified photoresist that are fabricated in accordance with the prior art, along with an example of a chemical amplification process used with of them.
However, there are problems associated with such chemically amplified photoresists. One problem is that such photoresists lack sufficient etch resistance, thermal stability, post exposure delay stability, and processing latitude to useful for fabricating ICs. In particular, while such photoresists are sufficiently transparent for deep UV radiation, they do not have the required etch resistance with respect to plasma etching that is evidenced by photoresists based on phenolic resins. This issue is compounded by the fact that the thickness of the photoresist is limited by pattern collapse issues at high aspect ratios, and the limited depth of focus of, for example, and without limitation, 193 nm photolithography tools. Because of this, as printed dimensions shrink, so too must the thickness of the photoresist shrink (for example, an aspect ratio of 3 or lower is necessary to prevent pattern collapse). However, lower photoresist thickness exacerbates the issue of etch resistance.
Another problem is that, after optical exposure and completion of the de-protection reactions, the photoresist in the exposed areas can shrink from ten to twenty percent. This is believed to be due to the loss of the bulky protecting groups as volatiles. This de-protection reaction does not occur in the unexposed areas. Thus, since the unexposed photoresist still contains resin with attached blocking groups, it is susceptible to shrinkage if these molecules are removed.
Due to the constraints of the photoresist design, and since the blocking groups are easily cleavable, the blocking groups can be removed by other reaction paths. One such reaction path entails thermal activation, where the photoresist is heated to a temperature that thermally breaks the bonds. This happens at different temperatures for different protection groups, but can be at a temperature as low as 40° C. to a temperature as high as 200° C. Thus, thermal activation may result in loss of blocking groups as volatiles, and a resulting shrinkage of the photoresist. The blocking groups can also be removed by other radiation sources including plasma discharges, or accelerated particles.
During electron beam exposure, a reaction similar to that occurring during optical exposure can occur. This reaction cleaves the link between the protecting groups and the resin, results in shrinkage of the photoresist. In addition, this reaction, and its associated photoresist shrinkage, is accelerated as the photoresist is heated by the energy of the incident electron beam. Since the full thickness of the photoresist is targeted for etch stabilization, substantial mass loss, and shrinkage, can result from the electron beam exposure. Further, because the interface between the photoresist and the substrate is constrained, the remainder of the photoresist shrinks in three dimensions. This leads to a phenomenon known as “pullback” where the top of the photoresist layer shrinks relative to the bottom. This effect is most pronounced on lithographic features such as contacts, line ends, and feature corners. The pullback phenomenon has undesired effects on features, which make them unacceptable for device fabrication.
Many different formulations of chemically amplified photoresists utilized for 193 nm exposure have been developed. Some examples of materials used for 193 nm lithography are disclosed in U.S. Pat. No. 6,319,655. For the next generation of lithography, new photoresist materials sensitive to 157 nm irradiation will be utilized for this application. Some of these materials are listed in an article entitled “Organic Imaging Materials, A View of the Future” by Michael Stewart et al., J. Phys. Org. Chem., 2000; 13: pp. 767–774; an article entiled “157 nm Resist Materials: Progress Report” by Colin Brodsky et al., J. Vac. Sci. Technol., B 18(6), November/December 2000, pp. 3396–3401; and in an article entitled “Synthesis of Siloxanes and Silsesquioxanes for 157 nm Microlithography” by Hoang V. Tran, et al. Polymeric Materials: Science & Engineering, 2001, p. 84 (all of which articles are incorporated by reference herein). However, due to the volatility of additives in the disclosed photoresist materials, electron beam exposure causes expulsion of these additives, which, in turn, causes shrinking of the photoresist.
Several attempts have been made to solve the problem of lack of etch resistance of chemically amplified photoresists. One attempt to improve the etch resistance of such photoresists based on methacrylate polymers involved introducing cycloaliphatic groups into the methacrylate polymers. Although this improves the etch resistance, it does not do so to a required extent. One proposal to improve the etch resistance does so after a photoresist coating has been exposed and developed. For example, various strategies have been suggested involving the chemical derivation of patterned resist films by reaction with a gas or liquid phase reagent imparting increased etch resistance. Significant changes have been noted in processes resulting in the incorporation or doping with various metallic precursors, for example by reaction of patterned resist films with alkyl compounds of magnesium or aluminum (see U.S. Pat. No. 4,690,838). However, this is problematic because the use of metal-containing reagents introduce undesirable contaminants which are difficult to remove using conventional stripping procedures. U.S. Pat. No. 6,319,655, which is incorporated herein by reference, describes a process for improving the etch resistance of chemically amplified photoresists, and in particular 193 nm photoresists, using a large area electron beam exposure. Electron beam exposure of chemically amplified photoresists, and in particular 193 nm sensitive photoresists, has been shown to improve the etch resistance and thermal stability of these photoresists.
In addition, many attempts have been made to correct or eliminate the problem of photoresist deformation or pullback using different process steps with an electron beam exposure. One such attempt to minimize shrinkage used lower current density electron beam exposures; another such attempt to minimize shrinkage utilized surface curing of the photoresist (i.e., by lowering the energy of electrons such that only an upper portion of the photoresist received the electron beam exposure); still another such attempt to minimize shrinkage utilized higher doses of electrons aimed at a lower portion of the photoresist coating relative to an upper portion; and still yet another such attempt to minimize shrinkage utilized a lower flux electrons with longer exposure times to reduce the temperature of the photoresist during the electron beam exposure to no more than 50° C. to minimize photoresist heating effects. In addition, different formulations of photoresist have been utilized in an attempt to minimize expulsion of photoresist components, and thereby, shrinkage. All of these attempts have failed at reducing the pullback effect caused by an electron beam hardening process, see FIGS. 2–4.
In light of the above, there is a need for a photoresist, and a method and apparatus for treating it to address one or more of the above-identified issues.